I. Field of the Invention
The present invention pertains to electric arc furnaces. More particularly, the present invention concerns gaseous atmospheres for electric arc furnaces. Even more particularly, the present invention concerns inert atmospheres for electric arc furnaces.
II. Prior Art
The manufacture of steel and alloys thereof, such as stainless steel, is well documented. There are a plurality of methods used to prepare iron and alloys thereof, such as steel, stainless steel as well as esoteric iron alloys. Steel, differs from pig iron and other forms of iron which are ordinarily manufactured, by a reduced carbon content. As is known to those skilled in the art to which the present invention pertains there are a plurality of methods currently known for the manufacture of steel. One such method is the open hearth method wherein pig iron, along with a certain amount of scrap iron, is deposited in a furnace hearth wherein a gas and air mixture is burned over the iron. The gas burned in the furnace generates a waste gas which are discharged up a chimney flue. However, before being discharged gases give off a considerable proportion of their heat to brick-lined heating chambers. The chambers, which are formed of refractory bricks, are, thus, heated to red hot temperatures. After the bricks reach the elevated temperature the gas flow is reversed and the mixture of gas and air is admitted through the heating chambers from which it absorbs heat. This preheating of the gas and air enables the combustion temperature of the flame to be considerably raised. The burning of the gas and air mixture above the pig iron causes an oxidation reaction to produce the steel.
Another method of steel manufacture is the Bessemer process. Yet another method is the oxygen steel-making process. Another form of steel manufacture, wherein mild steels of exceptionally high purity are manufactured, is the electric process. According to this process there is no air employed. In steel making with electric arc furnaces the requisite heat for the refining of the steel (sometimes pig iron) is supplied, not by the burning of gas or coal, but by an electric current. The heat is produced by an electric arc which is formed between a number of carbon electrodes and the surface of the molten bath. Rather than employing a combustive air for oxidizing the undesirable admixtures in the furnace, iron oxides are added, which give off their oxygen. In carrying out an electric arc steel manufacturing process an inert atmosphere is employed within the furnace.
The electric arc furnace operates on the principle of electric arc discharge. Ordinarily, this is allied to a gas discharge which takes place when electricity is passed through rearified gases. The arc discharge occurs when two carbon electrodes are brought into contact with each other and are then moved apart. Ordinarily, just before the carbon rods separate and direct material contact between them is broken, a high electric resistance is developed to the extent that the tips of the carbon electrodes begin to glow. This is associated with the emission of electrons which, because of the high emission temperatures, produces a high degree of ionization of the air. As a result of this ionization, the air in the immediate vicinity of the carbon tips becomes conductive to electricity, so that the current will continue to flow when the electrodes are no longer actually touching each other. The bombardment of electrons to which it is exposed causes the positive elctrode, in particular, to become white hot and a crater forms at its tip. In the actual arc itself, the gas molecules of the air dissociate. In electric furnaces, per se, the intense heat developed by the arc discharges utilized for the melting of metals. If the material to be melted is a poor conductor of electricity, the heat radiated by the arc formed between two carbon electrodes is used to melt it. On the other hand, if the material does conduct electricity, then the arc discharger may either be passed directly from the electrodes to the material or the electrodes may actually be buried in the material. In either case though, the considerable heat developed in the electrodes helps the current to generate heat in the material and thus attain the melting temperature. In the manufacture of steel, however, because it is a good conductor of electricity, the electrodes are placed above the material, i.e., the pig iron or steel.
Ordinarily, an inert atmosphere of nitrogen is employed in electric arc furnaces. By utilizing an atmosphere of rarified gas, having a higher heat retention content, it is reasoned that a longer electrode life with a decrease of energy input as well as a resulting higher quality molten metal and faster melt down time can be achieved. The present invention, as will subsequently be detailed, seeks to provide to improve inert atmospheres for conducting electric arc furnace manufacture of steel.